The present invention relates to a fiber-reinforced composite hollow structure, and a method and an apparatus for manufacturing the same. Particularly, this invention relates to a fiber-reinforced composite hollow structure which can be suitably used as a structural member with reduced weight, increased strength, and enhanced dimensional accuracy, and especially, to a fiber-reinforced composite hollow structure which is a composite hollow structure used preferably as a scaffold board for temporary construction works, and also to a method and an apparatus for quickly manufacturing the same with a high efficiency.
(1) Supports, called battens, are used for installation of concrete forms in construction works or civil engineering works. In general, battens are made from steel or aluminum; however, they are heavy, and susceptible to rusting and adhesion of concrete. Because of these problems, some of battens are formed of FRP-made pipes with reduced weight, increased strength, and enhanced corrosion resistance.
Also there is a tendency to use FRP hollow members molded by pultrusion as structural members for various structures such as columns, posts, fences and scaffold boards. In such a FRP hollow member, one side or diameter in cross-section is increased over 60 mm or more. When pultruding of a FRP hollow member having such a large cross-section is implemented, if the pultruding is performed using a curing die, there may occur problems such as: the die is enlarged in size and complicated in structure; a large pultruder must be employed because of the significantly increased pultruding resistance; and the die cost becomes higher because of the need of increasing the die strength. Also, there is a problem such that the pultruding rate is as slow as 1 m/min or less.
As a FRP pipe, there is known a composite pipe, (for example, trade name xe2x80x9cComposexe2x80x9d, produced by UBE-NITTO KASEI Co., LTD.) which has a three-layer integrated structure including an inner tubular layer (hereinafter, referred to as xe2x80x9ccenter corexe2x80x9d) made from thermoplastic resin, an intermediate layer made from FRP, and an outer cover layer made from thermoplastic resin. Such a composite pipe having a three-layer structure does not use any curing die and is subjected to curing with its shape kept by the center core and outer cover layer, and consequently, it has an advantage since the curing/molding rate becomes fast, and it is economical because of the significantly reduced costs of parts such as for the die. Of these composite pipes, a square-type pipe has a rectangular-like sectional shape with one side being relatively large as 50 to 60 mm; therefore, to keep the compressive strength and bending strength, the respective wall thickness of the intermediate layer and the center core is set to have large value. This not only disregards the significance of the reduction in weight, but also exhibits points to be improved in terms of resource saving and cost efficiency, with an additional inconvenience that the dimensional accuracy of the hollow portion is insufficient.
Another problem is that if the curing temperature is higher than the thermal deformation temperature of a thermoplastic resin which forms the center core, the center core is liable to be softened and deformed, leading to deformation of the cross-section of the final product.
For a composite pipe with the dimension of one side increased over 60 mm, the wall thickness of the center core and the intermediate layer is required to be increased. In this case, since there occurs a problem that the cross-section is liable to be deformed, the thickness of the center core is required to be further increased for suppressing this problem. This is because when the thickness of the intermediate layer is increased, heat generation caused by curing of FRP becomes larger, with a result that the strength of the center core is reduced and the center core is liable to be deformed. Also the contraction caused by curing becomes larger, with a result that the side portion is liable to be deformed in a convex or concave shape. That is to say, the increased thickness of the intermediate layer results in larger heat generation due to curing of FRP causing temperature rise, whereby the center core is reduced in strength and is liable to be deformed.
In addition to the above heat generation and contraction caused by curing, the following may be taken as other deformation factors. That is, when an uncured intermediate layer composed of reinforcing long fiber impregnated with a liquid-state thermosetting resin goes out of a shaping nozzle, corners of the intermediate layer are less deformed but the sides thereof are expanded in a convex-shape outward because of a repulsive force of the sides of the center core having already been cooled and solidified (the sides of the center core are deformed by the passing-resistance and pressure for squeezing the thermosetting resin upon passage thereof through the shaping nozzle, and the repulsing force is generated because of the release of the applied pressure). The larger the length of one side of a composite pipe, the stronger this tendency becomes. For a composite pipe with the length of one side in a range of about 60 mm or less, such deformation can be eliminated by preparing a shaping nozzle in which the inner surface is curved to form a concave-shape so as to compensate for the deformation.
For a composite pipe with the length of one side being much larger, since the above deformation becomes larger, it becomes difficult to compensate for the deformation. A composite pipe having a circular sectional shape is less susceptible to deformation resulting from the above reasons; however, in such a circular composite pipe with a diameter of a hollow portion being larger, the wall thickness of the intermediate layer is required to be increased over 10 mm to maintain the strength of the pipe, resulting in increased weight. For a composite pipe having a rectangular sectional shape, deformation of the longer-side becomes particularly larger. To be more specific, the larger the ratio of long-side (width)/short-side (height), particularly, this ratio being over 1.5/1, the larger the deformation becomes. Then, it is difficult to compensate for such large deformation by curving the shaping nozzle. In general, it is desired to use, as a scaffold board for construction/civil engineering works, a FRP hollow material having a thickness of about 20 to 60 mm and a width of about 200 to 300 mm in terms of light-weight, high strength, high durability, and high electric insulation. However, since the width thereof is over 60 mm, the above-described deformation becomes larger, to thereby reduce the strength of the material and to deteriorate stability in laminating these materials to each other upon storage and transportation.
When the ratio of long-side/short-side is more than 1.5/1, the compressive strength thereof is also reduced. This is because even slight deformation in the width direction tends to cause longitudinal cracks in a FRP intermediate layer because of stress concentration, and to cause concentrated overload to be particularly applied at a central portion. Such an inconvenience can be improved by also arranging reinforcing long-fibers in the width direction using a glass cloth, glass mat or the like. However, in this case, the rigidity in the longitudinal direction becomes low, and the manufacturing cost become high because of the increased number of manufacturing steps and the increased unit-weight. A composite pipe may be considered having a ladder-like sectional shape in which a plurality of FRP legs for connecting upper and lower FRP planes are arranged in the longitudinal direction in addition to outer peripheral FRP portions arranged on the upper, lower, right and left sides. In this composite pipe, however, if it is manufactured using a curing die, the die structure becomes more complicated and the pultruding resistance becomes significantly larger. In case the composite pipe is formed to have a large square cross-section, the FRP legs can be arranged in a criss-cross structure or a truss structure having a hollow portion formed into a triangular shape in cross-section for improving the strength and the dimensional accuracy and reducing the weight. However, there has not been developed a method for industrially manufacturing such a composite pipe for the same reasons mentioned above.
The present invention has been made by taking into account that there has not been known any method for industrially and efficiently manufacturing a large sized fiber-reinforced composite hollow structure having one side or diameter over 60 mm, with increased strength and dimensional accuracy and reduced weight. Thus the first object of the present invention is to provide a fiber-reinforced composite hollow structure having a three-layer structure, which is capable of keeping the strength and rigidity even when the wall thickness is reduced, to thereby achieve lightweight, resource saving, to make less deformation of the sectional shape, and to provide a method of manufacturing the fiber-reinforced composite hollow structure.
(2) In order to obtain a fiber-reinforced hollow composite structure of the above-described three layer structure, a method of extruding thermoplastic resin to cover an uncured core portion having a center core covered with an intermediate layer of uncured FRP is implemented by: providing a cross head die; introducing the uncured core portion to the cross head die; and extruding the same for covering. One method (the draft type) is characterized by employing a draft die having a discharge port being relatively larger than the outer periphery of an uncured core portion to be covered, wherein the uncured core portion is allowed to pass through a central space of the die head without contact with the discharge port of the die head, and thermoplastic resin is drafted downward to cover the uncured core portion. Another method (the pressure type) is characterized by pre-heating the uncured core portion, bringing it into direct-contact with molten resin in the die head, and closely covering the cured core portion with the resin by pressuring upon extrusion.
The draft type is poor in adhesion between the thermoplastic resin and the uncured core portion, but has a variety of features, for example, to treat the uncured core portion by the same die lip even if the dimension of the uncured core portion is changed somewhat, and to change the covering thickness with the same die lip by varying the clearance of the die discharge port.
On the contrary, the pressure type is good in adhesion between the thermoplastic resin and the uncured core portion, but it has not the variety of features of the draft type. That is to say, if the dimension of the uncured core portion is changed, the same die lip does not allow the changed dimension, and it is necessary to change the die shape upon change in dimension. The same is true for the change in covering thickness.
In the present invention, a covering die of the pressure type cannot be used because the uncured resin is squeezed at a pressure and is cured by heat of the covering resin. To be more specific, for an uncured FRP core portion impregnated with a liquid resin having a low viscosity, such as an unsaturated polyester, and in an uncured state, the uncured resin is squeezed at a pressure which is applied for extrusion of molten resin and is foamed and cured by heat of the covering resin, to thereby deteriorate the surface state (for example, to give rise to lumps, irregularities, breakage of the covering film, and the like). In summary, to cover an uncured FRP core portion impregnated with a liquid resin, a covering die of the pressure type cannot be used and a covering die of the draft type is required.
However, in the case of covering a large-sized uncured core portion having a cross-section with one side or diameter (in cross-section) which is more than 100 mm, there arises a problem that a uniform covering cannot be obtained because of occurrence of unevenness in covering thickness and wrinkling. More specifically, since a large-sized uncured core portion is almost always molded using a manufacturing line installed in such a manner as to extend in the horizontal direction, molten resin tends to be drooped by the effect of gravity, which obstructs uniform covering.
In general, molten resin extruded from an extruder is cooled by ambient air until it covers the uncured core portion. Thus, if the uncured core portion has a large size, it takes a longer time for the uncured core portion to be extrudingly-covered with the resin because the pultruding rate becomes slow. As a result, the resin is excessively cooled and is not sufficiently drafted. This causes wrinkling of the covering layer and cavities between the covering layer and the uncured core portion. For such a large-sized uncured core portion, the covering rate is in a range of 2-3 m/min or less.
Additionally, a difference in cooling rate between two locations causes unevenness of covering thickness therebetween. The reason to such unevenness by cooling is that environmental air is heated by the uncured core portion extruded from the die, and this causes convection. If an uncured core portion of a flat shape is to be covered with resin using a circular draft die, there are differences between the amount of time required for the extruded resin to reach the covering points. This causes unevenness in temperature at the covering points, thereby making it difficult to uniformly cover the uncured core portion.
In addition to the above extrusion-covering method using a cross head die, there is known another method of covering a thermoplastic resin on a large-diameter pipe or a wide plate-like uncured core portion. This is done by extruding the resin into a sheet, and winding this sheet to the uncured core portion by rotating the latter, or instead, previously preparing a roll of this resin sheet, rotating the roll, and winding the resin sheet around the uncured core portion. A further method is known in which a plurality of resin sheets extruded using a plurality of extruders are wound with edges continuously overlapped, to form a cover layer.
These methods are disadvantageous in that it is difficult to ensure a uniform covering thickness because of the presence of seams (overlapped edge portions) of the resin sheet. As a result, it is required to carry out a post-treatment of cutting off, polishing, or hot-pressing the protruding seams. If a resin soluble in a solvent is used, the protruded seams can be removed by wiping them with a cloth impregnated with the solvent. In each case, however, there occurs another problem that marks of such post-treatments are liable to remain.
Accordingly, a second object of the present invention is to provide a hot-extrusion covering method capable of covering a large-sized uncured core portion having one side or diameter (in cross-section) of more than 100 mm with thermoplastic resin, while keeping its surface in a uniform state (with no wrinkling) by using a draft type covering die, and to provide an apparatus therefor.
(3) In molding plastic products by profile extrusion, a water-cooled sliding type sizing method using a sizing nozzle (sizing plate made from a metal having a high thermal conductivity such as brass) has been generally adopted. In particular, a vacuum sizing method using a sizing nozzle inserted in a water bath surrounded by a vacuum atmosphere has been suitably used.
In manufacturing a three-layered composite hollow structure having two or more of center cores between which legs made from FRP are formed, after an uncured resin core portion is covered with a thermoplastic resin (ABS resin) using a covering die (cross head die), the core portion thus covered with the resin is required to be subjected to molding (sizing) for eliminating deviations among the center cores and smoothly finishing the surface. In this case, the above water-cooled sliding type sizing has the following problems.
For a large-sized composite hollow structure having one side (width) of more than 100 mm (peripheral length: 200 mm or more), there may arise an inconvenience that rippling waves-like wrinkles occur on the surface because of the increased sizing resistance. In particular, the sizing resistance tends to be increased in the case of indirect water-cooling sizing in which the sizing device is indirectly cooled by water. Additionally, only a slight change in dimension (outer shape) of a composite hollow structure brought about by change in condition may cause the structure to be stopped in the sizing device. This makes it impossible, in the worst case, to pultrude the composite hollow structure.
In the water-cooled sliding type sizing, if the covering thickness is uneven and/or foreign materials appear on the surface, the sealing-state of water between the composite hollow structure and the sizing device may be broken. This may leading to occurrence of water running, and irregularities may appear on the surface of the uneven portion because the cooling rate of this portion is different from those of the surroundings. Once water running starts, such running is expanded, and finally there arises a problem that water is scattered to the covering die, to make it impossible to continue the molding. For a large-sized composite hollow structure having one side of more than 100 mm, since the manufacturing speed (pultruding speed) becomes naturally slow, it is difficult to prevent water from running to the side of the covering die with viscosity of water caused between the surface of the sizing surface and the composite hollow structure. Such a phenomenon is liable to occur, particularly, at four corners and portions (the leg portions) between the center cores.
If sizing is implemented by making use of evacuation, suction of air and leakage of cooling water occur intermittently. The sizing apparatus has no flexibility to adapt to the varied dimension of a composite hollow structure being subjected to sizing. One set of sizing devices must be basically prepared for one shape (one kind) of a composite hollow structure. That is to say, the set of sizing devices, which are not commonly used for composite hollow structures different in width, thickness and the like, are poor in degree of freedom of usability.
After being covered with resin and subjected to sizing, the uncured resin core portion is required to be cured. For a composite pipe having a three-layer structure, there is preferably adopted, in terms of reliability, economical efficiency and safety, a method of curing the uncured resin core portion using hot water at about 100xc2x0 C. The shape of the uncured composite hollow structure having a three-layer structure is held by a thermoplastic resin covering layer (preferably made from a styrene based resin such as an ABS resin in terms of physical property and cost) formed on the outer periphery. However, as the temperature of the resin becomes high, the shape retention characteristic thereof becomes low (the thermal deformation temperature of the general ABS resin is about 90xc2x0 C. or less), causing the tendency that the sectional shape of the composite hollow structure is changed. Since the composite hollow structure is cured in hot water, the structure, particularly, having a large-size is applied with large buoyancy, and thereby it is liable to be deformed (warped) in the longitudinal direction. As a result, a means is needed to support the shape of a composite hollow structure from the outer side until the uncured core portion is cured. To prevent the floating of a composite hollow structure due to buoyancy, pressure rollers only for pressing down the composite hollow structure have been disposed in such a manner as to be spaced from each other at equal intervals (1.0 to 1.5 m). In this way, the warping in the longitudinal direction is reduced. However, warping may occur in the width direction (deformation of the sectional shape, which projects downward or is recessed upward).
Accordingly, a third object of the present invention is to provide a method capable of improving the surface smoothness, the accuracy of sectional shape, and adhesiveness of FRP/outer layer of a large-sized fiber-reinforced composite hollow structure having one side or diameter of more than 100 mm, and also capable of improving productivity of a composite hollow structure, and to provide an apparatus therefor.
(4) The fiber-reinforced composite hollow structure of the present invention is used not only as a scaffold board for construction works or civil engineering works, but also as a temporary scaffold board for electric work, painting work or the like, a walk board for culturing, or a permanent standing board for a circuit way, a promenade or overpath.
When used outdoors as a temporary scaffold board or the like, the fiber-reinforced composite hollow structure of the present invention often gets wet from rain. In this case, the composite hollow structure of a three-layer structure including a center core made from thermoplastic resin, an intermediate layer made from FRP, and an outer cover layer made from thermoplastic resin exhibits an inconvenience that the smooth outer cover layer becomes slippery when it gets wet. Also, in case a powder-like material, such as sand or other such material, adheres to the covering layer, it tends to get slippery.
Accordingly, a fourth object of the present invention is to provide a fiber-reinforced composite hollow structure which is less slippery than a conventional one even if it gets wet or particles such as sands adheres thereto, and which is suitably used as a scaffold board for various works.
(5) When used as a scaffold board for various works, a fiber-reinforced composite hollow structure may possibly fall or collide into a wall or the ground during transportation, or comes in contact with other scaffold boards in use, so that the impact force applied to the structure may cause cracks in the FRP-made intermediate layer of the structure at its end portion. The progress of these cracks leads to longitudinal cracks in the intermediate layer and reduction in strength and rigidity of the structure, thereby making, in the worst case, it impossible to the continue use of the structure. On the other hand, provision of a metal-made protective member on the end portion of a composite hollow structure cause a problem in terms of electric insulation and corrosion resistance. To cope with such a problem, it may be considered to form the end portion of the structure by insert-molding of thermoplastic resin or thermosetting resin. However, since a composite hollow structure of the present invention is generally long in length and has a continuous hollow portion, insert-molding of resin cannot be applied.
It may be considered to fit a resin-made cap in a hollow portion of a center portion. It is preferred to fix such a cap in the hollow portion of the center core using an adhesive. However, in this case, permeation of water into the hollow portion cannot be fully prevented because the adhesive cannot be fully applied around the portion of the cap to be fitted to the hollow portion, and even if the adhesive is fully applied, there is a possibility of partial leakage of water after a long-term use. In the case of using a hermetic-type cap, if water is accumulated in the hollow portion of the center core, it cannot be easily removed, which may degrade the electric characteristics and safety of the structure. For this reason, it is preferred to provide drain holes in the cap. If the composite hollow structure of the present invention is used as a scaffold board, since the scaffold board is designed with its function kept constant if the board is turned over, the drain holes must be designed to maintain their function even if the board is turned over.
If the cap is a type which is mounted to each hollow portion of the composite hollow structure, and if the structure has, for example, seven pieces of the hollow portions, 14 pieces of the caps must be mounted on both ends of the hollow portions. This will certainly complicate the mounting work of the caps. Further, if there are differences and/or inclinations among the hollow portions, the adjacent caps may be in contact with each other, thus making it, in the worst case, impossible to mount the caps. Even in the case of using an integral-type cap structure in which seven pieces of projections as plugs are integrally juxtaposed, there is a fear that the projections cannot be inserted into the hollow portions unless they exhibit a deformation function. If the caps are not deformable and if the projections are designed to be small in consideration of dimensional tolerances of the composite hollow structure, there is a fear that the projections will float and will not be brought into contact with the walls of the hollow portions of the composite hollow structure. In this case, there is a fear that the projections tend to fall off from the hollow portions. Even if the number of the projections is reduced, and only three projections are provided correspondingly to the right, left and central hollow portions, the above problem cannot be solved. Additionally, in a configuration in which the projections are not fitted in the hollow portions but a cap is disposed to surround the hollow portions as a whole, the thickness and width of the cap become larger and a step portion will be formed. As a result, the cap is possibly loosened upon transportation because the composite hollow structure will get caught to each another. Also, the composite hollow structures are usually stacked upon transportation or storage, but the workability will become poor, and the stacks will tilt and consequently collapse because of the presence of the step portions.
Accordingly, a fifth object of the present invention is to provide a fiber-reinforced composite hollow structure provided with a thermoplastic resin-made cap which allows water having been accumulated in a hollow portion of a center core to be easily removed therefrom, and which is easily fitted in the hollow portion and less removed therefrom, so as to eliminate occurrence of cracks in a FRP-made intermediate layer even if the structure falls or collides into a wall, the ground, and other composite structures, to thereby suppressing occurrence of longitudinal cracks in the intermediate layer to bring about improved longevity.